1. Field of the Invention
The present invention relates to a semiconductor device and also to a wire bonding method for connecting a first bonding point and a second bonding point by a wire and more particularly to a semiconductor device that has a particular wire loop shape and to a method for forming wire loop having such a particular shape.
2. Prior Art
As shown in FIGS. 5(a) and 5(b), in a semiconductor device, a pad 2a (first bonding point) on a semiconductor chip 2 mounted on a lead frame 1 and a lead 1a (second bonding point) on the lead frame 1 are connected by a bonding wire (called merely "wire") 3. The loop shape of the wire 3 in this case may be a trapezoidal loop shape as shown in FIG. 5(a) or a triangular loop shape as shown in FIG. 5(b).
Wire loop formation methods of this type are described in, for example, Japanese Patent Application Publication (Kokoku) No. 5-60657 and Japanese Patent Application Laid-Open (Kokai) No. 4-318943.
The trapezoidal loop shown in FIG. 5(a) is formed by the process shown in FIG. 6.
In step (a) shown in FIG. 6, a capillary 4 is lowered so that a ball (not shown) formed on the tip end of the wire 3 is bonded to a first bonding point A. This is done while a damper (not shown), which is located above the capillary 4 and can hold the wire when closed and feed the wire when opened, is opened. After this, the capillary 4 is raised to point B delivering the wire 3.
Next, as seen in step (b), the capillary is moved horizontally in the opposite direction from the second bonding point G to point C. Generally, to move the capillary 4 in the direction opposite from the second bonding point G (for forming a loop in the wire) is referred to as a "reverse operation". Because of this reserve operation, the wire 3 assumes a shape that extends from point A to point C; and as a result, a kink 3a is formed in a portion of the wire 3. The wire 3 delivered in the process from point A to point C forms the neck height portion 31 of the loop shown in FIG. 4(a).
Next, in step (c), the capillary 4 is raised to point D while delivering the wire 3.
Afterward, as shown in step (d), the capillary 4 is again moved horizontally to point E in the opposite direction from the second bonding point G, i. e., another (or second) reverse operation is performed. As a result, the wire 3 assumes a shape inclined from point C to point E, and a kink 3b is formed in a portion of the wire 3. The wire 3 delivered out of the capillary 4 in the process from point C to point E forms the trapezoidal length portion 32 shown in FIG. 5(a).
Furthermore, in step (e), the capillary 4 is raised to point F while delivering the wire 3. The amount of wire 3 delivered is equal to the inclined portion 33 shown in FIG. 5(a). Afterward, the damper (again, not shown) is closed. Once the damper is closed, the wire 3 is not delivered even if the capillary 4 subsequently is moved.
As shown in steps (f) and (g), the capillary 4 performs a circular-arc motion (or a circular-arc motion followed by a straightly lowering motion) so that the capillary 4 is positioned at the second bonding point G, and the wire 3 is bonded to the second bonding point G, thus connecting the first and second bonding points A and G.
On the other hand, the triangular loop shown in FIG. 5(b) is formed by the process shown in FIG. 7.
In this triangular loop formation, the trapezoidal length portion 32 described in the above loop formation is not formed. Accordingly, the second reverse operation in step (d) in FIG. 6 is not performed. Thus, the steps (c), (d) and (e) in FIG. 6 are replaced by the single process as shown in step (c) of FIG. 7. In particular, the steps (a) and (b) are the same as the steps (a) and (b) shown in FIG. 6, respectively; and after the first reverse operation in step (b) of FIG. 7, the capillary 4 is raised to point F while delivering the wire 3 in step (c). Afterward, the capillary 4 performs the operations steps (d) and (e) in the same manner as the operations done in the steps (f) and (g) shown in FIG. 6, so that the wire 3 is bonded to the second bonding point G.
As seen from the above, the triangular loop formation shown in FIG. 7 is simpler than the trapezoidal loop formation shown in FIG. 6 and is therefore advantageous in that the loop formation is performed in a shorter time. However, in cases where the height difference between the first bonding point A and the second bonding point G is large, or in cases there is a large distance between the first bonding point A and the edge portion of the semiconductor chip 2, the wire 3 tends to come into contact with the edge portion of the semiconductor chip 2 when the triangular wire loop shape as shown in FIG. 5(b) is formed. In such cases, the trapezoidal wire loop formation is employed so as to avoid the contact between the wire 3 and semiconductor chip 2.
In the trapezoidal loop formation process shown in FIG. 6, the first reverse operation shown in step (b) is performed with the capillary 4 in a position which is at a height close to the height of the first bonding point A. Accordingly, the kink 3a is relatively strong. However, the second reverse operation shown in step (d) is performed with the capillary 4 in a high position which is far away from the first bonding point A. Accordingly, the kink 3b is difficult to form and is unstable. As a result, the portion of the wire in the vicinity of the kink 3b (see FIG. 5(a)) tends to unstable and has a weak shape-retaining strength, and therefore, this portion of the wire in the vicinity of the kink 3b may rise up or drop downward. If the shape-retaining strength of the portion of the wire near the kink 3b is weak, the wire may bend when pressure from the outside is applied thereon. For example, wire bending may easily be caused by external forces such as shocks or vibration of the wire 3 due to capillary contact or ultrasonic oscillation during bonding to the second bonding point G, or mold flow due to the injection of molding material during molding, etc.